In recent years, efforts have been directed toward both automating and miniaturizing chemical and biochemical reactions. The lab-on-a-chip and biochip devices have drawn much interest in both scientific research applications as well as potentially point-of-care applications because they carry out highly repetitive reaction steps with a small reaction volume, saving both materials and time. While traditional biochip type devices utilize micro- or nano-sized channels and corresponding micropumps, microvalves, and microchannels coupled to the biochip to manipulate the reaction steps, these additional components increase cost and complexity of the microfluidic device.
Digital microfluidics (DMF) has emerged as a powerful preparative technique for a broad range of biological and chemical applications. DMF enables real-time, precise, and highly flexible control over multiple samples and reagents, including solids, liquids, and harsh chemicals, without need for pumps, valves, or complex arrays of tubing. In DMF, discrete droplets of nanoliter to microliter volumes are dispensed from reservoirs onto a planar surface coated with a hydrophobic insulator, where they are manipulated (transported, split, merged, mixed) by applying a series of electrical potentials to an embedded array of electrodes. Complex reaction series can be carried out using DMF alone, or using hybrid systems in which DMF is integrated with channel-based microfluidics. Hybrid systems offer tremendous versatility; in concept, each reaction step can be executed in the microfluidics format that best accommodates it.
For many applications it is most convenient to carry out DMF on an open surface, such that the matrix surrounding the droplets is ambient air. However, use of the air-matrix format necessitates accounting for droplet evaporation, especially when the droplets are subjected to high temperatures for long periods of time. In some instances, evaporation is considered a desirable feature, as it can facilitate concentration and isolation of solutes of interest. In biochemical contexts, however, evaporation frequently limits the utility of air-matrix DMF, because enzymatic reactions are often highly sensitive to changes in reactant concentration. Largely for this reason, investigators have attempted to use oil-matrix DMF for biochemical applications, despite numerous drawbacks including: 1) the added complexity of incorporating gaskets or fabricated structures to contain the oil; 2) unwanted liquid-liquid extraction of reactants into the surrounding oil; 3) incompatibility with oil-miscible liquids (e.g., organic solvents such as alcohols); and 4) efficient dissipation of heat, which undermines localized heating and often confounds temperature-sensitive reactions.
Another strategy is to place the air-matrix DMF device in a closed humidified chamber, but this often results in unwanted condensation on the DMF surface, difficult and/or limited access to the device, and need for additional laboratory space and infrastructure. These issues may be avoided by transferring reaction droplets from the air-matrix DMF device to microcapillaries, where they can be heated in dedicated off-chip modules without evaporation problems, however, this complicates design and manufacture of the air-matrix DMF device, introducing the added microcapillary interfaces and coordination with peripheral modules.
It would be highly advantageous to have an air-matrix DMF device that avoids the difficulties of evaporation even when droplets are heated or exposed to otherwise evaporative conditions, without requiring removal of the droplets from the matrix, while ensuring that proper concentrations and overall kinetics is maintained. Described herein are methods and apparatuses, including systems and devices, that may address the issues raised above.